Further studies on the rat jejunal ADase variants which characterize mitotically active versus differentiated epithelial cells will be carried out. Each variant will be purified to homogeneity, and the nature of the apparent molecular weight difference clarified by SDS gel electrophoresis, peptide mapping, and in vivo tracer experiments. Critical kinetic parameters will be re-evaluated. The subunit structure of the various high molecular weight ADase proteins from normal and neoplastic tissues will be further evaluated to clarify how the conversion factor protein interacts with the catalytic subunit. Inter-subunit crosslinking and electron microscopy will be employed. A similar study will be undertaken with respect to the free conversion factor. With the use of antisera to the purified red cell enzyme, radioimmune assay will be employed to establish the presence of cross-reacting protein in SCID patients. These proteins will be purified and studied for genetic alterations. The structure of the SCID inhibitor will be further elucidated. The microheterogeneity characteristic of the human enzyme will be studied with respect to differences in amino acid composition, carbohydrate content, and peptide mapping.